Current detecting system



. 3,170,070 CURRENT DETECTING SYSTEM Walter F. Schuehard, Hingham, Mass, assignor to S. H.

Couch Company, Inc., North Quincy, Mass, a corporation of Massachusetts Filed Oct. 19, 1959, Ser. No. 847,219 1 Claim. (Cl. 307-885) The present invention relates in general to transistor circuits and more particularly concerns a novel current detector which offers an exceptionally low impedance path to the current it detects, the path being capable of carrying relatively high values of current while dissipating negligible power. In addition, the novel circuit provides an indication of the presence of current over an exceptionally wide range of magnitudes.

The invention is especially advantageous for unambiguously indicating the presence of a load current, regardless of the current magnitude. For example, consider a hospital call system in which each patient has a call switch in series with a light above his door or bed for requesting attention from the nurse on duty. The supervising nurse is normally at a central location. A signal light or buzzer alerts the nurse'whenever one or more patients request aid by pressing call buttons. While a relay system may be employed for providing the desired indication, the relay which controls indicating device operation is subjected to frequent operations. As a result, contact wear and other causes limit the duration of reliable operation.

It is an important object of the present invention to provide a low impedance current responsive device capable of reliably and unambiguously indicating the presence of a load current over a Wide range of current amplitudes'with maximum efiiciency.

Another object of the invention is to sense the presence of a load current with a device which consumes negligible power.

Still another object of the invention is to sense the presence and polarity of a load current in accordance with the preceding objects.

According to the invention, a transistor having at least base-collector and base-emitter junctions is arranged with the former junction forward-biased and the latter junction reverse-biased. The full load current is drawn through the base of the transistor. Yet, relatively large currents are carried with a minimum amount of power dissipation.

This occurs because the area of the base-collector junction is relatively large. As a result of this large area and the application of a forward bias across the junction, its impedance is very low so that the potential drop is very low, even at high currents. By reverse-biasing the baseemitter junction, carriers from the collector region which flow into the base region are attracted across this junction to the emitter electrode to indicate the flow of a load current. As a result of the base-emitter junction being relatively small in area, the emiiter current reaches saturation for relatively small values of load current. Thus, the. power dissipative capabilities of the baseemitter junction are'not exceeded despite wide variations in the load current drawn through the base.

Other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing in which:

FIG. 1 shows the current relationships in a reversebiased transistor;

FIG. 2 shows a schematic circuit diagram of a system "ice FIG. 3 shows a schematiccircuit diagram of another system according to the invention; FIG. 4sho-Ws a typical relationshipbetween emitter current and the ratio of emitter current to base current as a function of base current for a high value of emitter resistance in a circuit arranged according to the invention; and,

FIG. 5 is a graphical representation of emitter current as a function of base current for lower values of resistance in series with the emitter.

With reference now to the drawing and more particularly FIG. 1 thereof, there is shown a PNP transistor T1 which is reverse-biased; that is, the base-collector junction is forward-biased while the base-emitter junction is reverse-biased. This is accomplished by maintaining base terminal 11 negative with respect to collector terminal 12 and emitter terminal 13 negative with respect to both terminals 11 and 12. For an NPN transistor, establishing relative polarities of opposite sense would reverse-bias such a transistor.

Current then flowsfin the direction indicated by the arrows. The algebraic relationship between the magnitudes of the collector current l thevbase current 1,; and the emitter current I is With this biasing arrangement, a number of important results are obtained. The area of the collector-base junction is relatively large. As a result, by forward-biasing this junction, arelatively large current flow maybe established across the junction with a relatively low potential. That is, the forward resistance of the junction is very low.

It has been discovered that by reverse-biasing the emitter-base junction, a very high base current may be established without burning out the transistor. Evidently the reason for this is that while the current flow is heavy, the potential drop across the collector-base junction is so low that the product of potential and current is well below the power-dissipation capability of the transistor. In addition, it has been discovered that by reversebiasing the base-emitter junction, the emitter current is essentially constant, once a relatively small value of base current has been established.

It has been discovered that for a typical alloy junction transistor, such as the CBS type 2N256, the base current can be equal to or greater than the rated collector current of the transistor without damaging the transistor while establishing the relatively small value of emitter current. A possible explanation for the nearly constant current characteristics of the reverse-biased emitter junction for such a wide range of base currents concerns the relatively small area of the base-emitter junction. It seems as if a small base current injects enough emitter region minority. carriers opposite the base-emitter junction so that the full capacity of the latter junction to pass such carriers into the emitter region is utilized. 7

The properties of the reverse-biased transistor may be advantageously employed as a current detector capable of detecting the presence of either a small or very large load current without introducing any appreciable voltage drop in the conductor supplying the load current. Referring to FIG. 2, there is illustrated a schematic circuit diagram showing a variable load, such as might occur with a lamp annunciator where one or more lamps may be ON concurrently, in series with the base of a reverse-biased power transistor.

Each load is represented by resistors designated L L connectable in parallel between the base terminal 11 of transistor T 'and the negative bus '14 by associated switching means S S respectively. Collector terminal 12 is maintained positive with respect to bus 14. A resistance R in series with the reverse-biased base-emitter junction represents the resistance of a detecting device such as a signal lamp, buzzer, relay, meter or other device responsive to the flow of emitter current.

By suitably choosing the value of the resistance R the emitter current remains substantially the same whether one or all of the loads are drawing current; yet, the voltage drop across the forward-biased base-collector junction is of the order of only a few millivolts.

Referring to FIG. 3, there is illustrated a schematic circuit diagram of a current detecting system in accordance with the invention capable of detecting not only the presence of a load current, but also its polarity. The lower part of the circuit is essentially the circuit of FIG. 2. The upper part of the circuit is essentially the same as the lower part with the exception that transistor T is a reverse-biased NPN transistor. Elements in the upper portion are identified by appending a prime to the same reference symbols used to identify corresponding elements in the lower portion.

The bases of the two transistors are connected together and to the same line 11. The collectors of the two transistors are connected to the same terminal 12. A detecting device representable by the resistance R is connected between the emitter of transistor T and line 15, the latter line being maintained at a potential which is positive with respect to lines 11 and 14. A number of loads L L are selectively connectable in parallel between lines 11 and by associated switches S '-S Line 11 will then carry the base currents I and T of transistors T and T respectively, these currents being of opposite polarity. The flow of current through any of the lower loads will be indicated by a flow of emitter current through the resistance R while the flow of current through any of the upper loads will be indicated by a flow of current through the resistance R In addition, due to the low impedance of the forward-biased base-collector junctions, the potential on terminal 12 is very nearly the same as that on line 11 and a measure of this potential may be used for null detection. That is, the potential on terminal 12 with all loads disconnected is the same as the potential on line 11 when equal currents I and I are being drawn by the upper and lower loads then connected. The circuit additionally has utility as a null detector for sensing very slight differences in load currents being drawn.

Referring to FIG. 4, there is illustrated the relationship between emitter current and base current for a value R of 10,000 ohms in the circuit of FIG. 2 for a 2N256 transistor with the potential between terminal 12 and bus 14 being 10 volts. The ratio of emitter current to base current, ,B as a function of base current is also graphically represented. Observe that once the base current reaches a value somewhat less than a milliamp, the emitter current remains essentially constant.

In FIG. 5, the emitter current as a function of base current is graphically represented for lower values of the resistance R Lowering this resistance only serves to increase the emitter saturation current and the base current value which must be exceeded to cause emitter current saturation.

The maintenance of constant emitter current for widely varying load currents is especially useful where the indicator R is a light bulb since the indicating bulb may burn with constant brightness, regardless of load current, without exceeding its ratings.

The circuitry and techniques according to the invention provide reliable means for efliciently detecting the presence or absence of a widely varying load current with an exceptionally compact physical arrangement which may be constructed without moving parts to maximize reliable operating life. In addition, the invention may be adapted to indicate the sense of current flow and be used for null detection. Numerous other uses and modifications of and departures from the specific embodiments described herein may now be practiced by those skilled in the art without departing from the inventive concepts. Consequently, the invention is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

A transistor circuit comprising, first and second complementary transistors each having at least a base-emitter rectifying junction and an oppositely polarized emittercollector rectifying junction, the bases of said transistors being connected together, the collectors of said transistors being connected together, first and second lines, first and second means respectively in series with said first and second transistor emitters and said first and second lines respectively for detecting emitter current of the respective transistors, means for maintaining said first and second lines at potentials relative to the potential on said connected together collectors so as to forward-bias said basecollector junctions and reverse-bias said base-emitter junctions, first and second groups of loads, and switching means for selectively connecting each load from said first group in series with said first line and said connectedtogether bases and each load from said second group in series with the latter bases and said second line so that emitter current flows across a reverse-biased base-emitter junction in response to load current being drawn across the adjacent forward-biased base-collector junction.

References Cited in the file of this patent UNITED STATES PATENTS 2,666,818 Shockley Jan. 9, 1954 2,848,653 Hussey Aug. 19, 1958 2,930,942 Levine et al Mar. 29, 1960 2,945,133 Pinckaers July 12, 1960 2,992,409 Lawrence July 11, 1961 2,997,606 Hamburger et al Aug. 22, 1961 3,097,307 Bonn July 9, 1963 FOREIGN PATENTS 1,158,630 France June 17, 1958 216,085 Australia July 15, 1958 596,394 Canada Apr. 19, 1960,

OTHER REFERENCES Hurley: copyright 1958, John Wiley and Sons, Junction Transistor Electronics, Tk 7872 (Sect. 8.3, p. 149 relied on). 

